If your aim is to kill people, you DON'T want your victim to be armed. That's why these things take place in areas where people are defenseless. If they wanted violent resistance they would target places like military bases and or areas where there are more cops or armed guards.

What these people are looking for is to go out or get captured in a blaze of glory. They want a dramatic, violent response. Most are seeking "suicide by..." well anyone (cop, military, random person, etc.).